Continental PET Technologies, Inc. (CPT) developed and commercialized a sequential injection process for making multilayer plastic containers (see U.S. Pat. Nos. 4,550,043, 4,609,516 and 4,781,954). These containers are currently in use as hot-fill juice and ketchup containers. The CPT process enables the use of thin layers of expensive barrier materials (for oxygen sensitive products), external layers of thermal-resistant materials (for high temperature filling and/or caustic wash refill applications), and/or interior layers of recycled materials (e.g., core layers not in contact with the food product).
U.S. Pat. No. 4,781,954 describes CPT's sequential injection process for making a five-layer container having inner and outer layers of polyethylene terephthalate (PET), a central core layer of PET, and first and second intermediate layers of a barrier polymer. The intermediate layers can be made very thin, e.g., 0.01-0.15 mm, based upon the relative melting points of the different polymers and the layer solidification/tunnel flow characteristic of the sequential process--wherein later-injected molten polymers push prior-injected molten polymers between outer layers which have solidified on the cold mold cavity and core walls. More specifically, a first metered shot of PET is injected into the end cap (via the injection gate or sprue) of the preform mold and flows about halfway up the sidewall where it momentarily slows or stops, before a second injection is made. Inner and outer solidified layers of PET are formed along the cold mold cavity and core walls, while the interior PET remains warm and fluid. Then, a second metered shot of a barrier polymer is made through the gate, which forms a melt pool at the bottom of the preform. The flow resistance provided by the first shot (PET) in the sequential injection process has a self-leveling effect on the second shot, causing the second shot (barrier) to form a melt pool that is substantially evenly distributed at all points (360.degree.) around the circumference at the cavity end cap. Finally, a third metered shot of PET is made which pushes the barrier melt pool up the sidewall to form two thin intermediate layers adjacent the solidified inner and outer PET layers, with the molten PET core layer (third shot) there between. The barrier material (e.g., EVOH) normally has a lower melting temperature than the first-injected (PET) material, and therefore the cooling effect of the solidified first layers on the barrier material is not as great as the cooling effect of the mold surfaces on the first (PET) material. Thus, while there will be some solidification of the barrier material as it contacts the inner and outer solidified PET layers, the third injected (PET) material will remelt some of the solidified barrier material and advance it together with the remaining barrier melt material up through the center of the preform (tunnel flow), thereby further reducing the thickness of the intermediate barrier layers.
The result is a relatively simple and highly-reproducible process with a number of important benefits. For example, the five-layer PET/EVOH ketchup bottles made by this process have largely replaced the prior commercial polypropylene/EVOH/adhesive bottles, for at least three reasons. First, the five-layer PET/EVOH container is transparent. PET provides a sparkling clear container which is aesthetically superior to the prior translucent polypropylene container. Second, the EVOH layers in the PET container constitute only 1.5 percent of the bottle's weight and do not require adhesive layers to adhere the EVOH to the PET. Rather, the CPT process maintains the PET/EVOH layer relationship during manufacture and use, but allows the layers to separate readily when the bottle is reground for recycling; the two polymers are then separated by conventional gravimetric and other means and the PET reprocessed as part of the PET soda bottle recycling stream. In contrast, the prior polypropylene bottle utilizes about 6 to 10 percent EVOH barrier and olefinic adhesive layers which, not only are more expensive, but also prevent post-use segregation of the constituent polymers. As a result, most of these bottles end up in municipal waste dumps. A third important commercial benefit is that the PET/EVOH container (unlike the polypropylene version) is substantially shatterproof when dropped onto a hard surface. For at least the above reasons, the CPT container has been a significant commercial success and recognized by the industry with various design awards.
One problem that has vexed multilayer plastic container manufacturers, using both sequential and simultaneous injection processes, is an uncontrolled natural flow phenomena known as "backflow" which occurs in the terminal end of an injection-molded article during the packing and cooling stage of the injection cycle. This is described in U.S. Pat. No. 4,627,952 to Ophir, col. 1, lines 17-31, as an interruption in the laminar flow of the polymer in the mold cavity when it strikes the terminal end of the cavity and reverses its flow direction upon packing. As described by Ophir, in a conventional injection molding process one first injects polymer melt into a closed mold and, subsequently, packs additional melt into the cavity to compensate for the densification (shrinkage) of the melt during the cooling stage. In the terminal or "dead-end" zone of the mold cavity where the melt layers begin to pack, the polymer flow strikes the terminal wall and reverses its flow direction to produce "rebound wave patterns" on a molecular scale in the melt; these wave patterns are points of structural weakness because the multiple layers (waves) are liable to separate. Ophir's proposed solution is to open up the previous dead-end of the mold by providing an outlet, which allows the molten polymer to continuously exit through the terminal end of the mold.
Ophir's proposed solution of opening up the terminal end of the mold may be acceptable in certain applications, but it obviously increases the complexity of the mold apparatus and introduces new variables in the process, including the need to trim and remove "excess" terminal end material. Tighter controls over the temperature, pressure, viscosity, etc., may be another possible way to eliminate backflow; however, such controls reduce the "processing window" available to the container manufacturer and thus inherently increase the manufacturing cost and/or number of defective containers. This is particularly true with today's large-scale multicavity injection molding systems, having a high throughput. Thus, there is need for a better understanding of the undesirable and substantially uncontrollable backflow phenomena which occurs during packing, and a method to avoid the same.
Another serious problem experienced by many multilayer container manufacturers is the inability to provide an effective foil seal at the top sealing surface of the container. For example, heat-bonded foil seals are used on commercial ketchup containers to seal out oxygen. Any deficiency in the seal between the top end of the container and the foil liner leads to exposure of the product to oxygen, with resulting degradation and/or leakage. Again, the causes and ability to control the defective seal are not yet well understood.